Small communication units such as pagers and portable radios utilize a sound transducer or speaker for converting electrical signals to sound. Sound transfer efficiency in these applications is a critical parameter since it affects the loudness or acoustic power delivered by the device to outside world.
FIGS. 1, 2 and 3 illustrate a typical sound transfer mechanism. A speaker grille assembly 700 includes, a speaker grille 510 having plurality of openings 540 in shape of circular holes or slots for the passage of sound. A porous felt pad 200, having uniform porosity (open area), is positioned behind the grille 510 for preventing entry of foreign material into the unit. The speaker griller 510 is usually a part of a member 500 which may be a part of a housing assembly. A loudspeaker 300 is normally positioned behind the felt 200 and is fastened to the member 500.
Since the passages 540 are straight through holes, the rear surface ports 522 are equal to the front surface ports 511. The felt sheet 200 is usually positioned directly against the rear surface 520 thereby covering the rear openings 522 on the grille 510.
The disadvantage of the known art is that felt sheet 200 limits the open areas of ports 522 by its porosity ratio (open area) and reduces effective sound passage path thereby impeding acoustic power transfer. This configuration of the grille 510 and the felt sheet 200 assembly causes acoustic transmission losses in the range of 1.5 to 2.0 dB SPL (dB sound pressure level).